Heat flow through building components can be modified by the choice of materials. The main heat transfer process for solid, opaque building elements is by conduction. Thermal insulation, which provides a restriction to heat flow, is used to reduce the magnitude of heat flow in a ‘resistive’ manner. Since air provides good resistance to heat flow, many insulation products are based upon materials that have numerous layers or pockets of air trapped within them. Such materials are thus low density and lightweight, and, in most cases, not capable of giving structural strength. Generally, the higher the density, the greater the heat flow. Since structural components are often, of necessity, rather high in density, they are unable to provide the same level of resistive insulation. Warmth is a valuable commodity and it will seek every possible means to escape from a building. Walls, roofs, floors, chimneys, windows are all escape routes.